A Brief History of Time: Chapter 4 Summary & Analysis

In the early 1800s, Marquis de Laplace thought that because science was doing such a good job explaining everything, that it would be possible to predict everything if only scientists knew the total make up of the universe at one moment in time. He thought this could extend even to predicting human behavior. This idea remained influential for decades, though it was unpopular among those who believed God’s freedom to act should be uninhibited.

Later, Lord Rayleigh and Sir James Jeans suggested hot bodies, like stars, radiate energy at infinite rates; this would mean the total energy emitted would be infinite, however, which is not considered possible. Max Planck then suggested that light, and all waves, would be emitted in certain amounts, called quanta. Higher frequencies of light would be emitted in higher-energy quanta. This would make the energy released, and the energy the star lost, finite.

Werner Heisenberg used this theory to create his uncertainty principle. To measure a particle’s position and velocity, one must shine light on it to see where it is. The higher the frequency of the light in the quantum, the more accurately you can see the particle, because the wavelengths of the light will be shorter. But that means more energy will be applied to the particle, therefore changing its position or velocity. So, the more accurately one wishes to measure a particle’s position or velocity, the more uncertainty created. This is a fundamental principle of the world.

This was the end of Laplace’s idea of determinism. There was still place for God in this model, but it did not help mere mortals to understand how he might work. Instead, it was better to leave out of the theory that which humans cannot see.

Heisenberg, Erwin Schrodinger, and Paul Dirac in the 1920s created quantum mechanics based on the uncertainty principle. This theory does not predict definite outcomes, but potential outcomes. It works on the basis of probability, with no definite outcome for each individual observation, therefore introducing randomness into science. Einstein objected to this approach despite the fact his Nobel Prize was partly awarded for his contributions to the theory. He said, “God doesn’t play dice.” Yet quantum mechanics works very well with observations and it underlies all of modern science, including electronic chips. The only areas of science that not yet been integrated into this theory are gravity and the larger structure of the universe.

Planck suggested that light, although a wave, could act as a particle, being emitted only in certain quanta. Heisenberg’s uncertainty principle made particles seem more like waves, with their movement spread out according to probability. There is therefore a duality between waves and particles in this new theory. This means scientist must consider the interference of these waves, where the peak and a trough of two waves meet, canceling each other out. This same effect creates the colors in bubbles, as waves overlap and strengthen or cancel each other out.

If particles can be like waves, this canceling out happens with them also. If one passes light through two slits in a paper divide onto a wall behind, the light will generally travel different distances from the light source, and through the slits, to reach the wall. Therefore, the wavelengths of these beams of light will overlap rather than arrive “in phase with each other,” canceling out where a wave’s trough hits a peak, or strengthening each other where a peak combines with another peak. This creates a fringed pattern of light on the wall, as the light has not hit in a uniform manner. This happens with particles in the same way. But when passing just one particle through one slit at a time, the pattern still shows, as though it had passed through both slits at the same time and interfered with itself.

This has helped scientists to understand the atom. At first atoms were seen as mini solar systems, with a nucleus orbited by other miniscule particles, but many wondered why it did not all collapse. Niels Bohr suggested in 1913 that electrons could only orbit at specific distances, which would balance it all out. According to quantum mechanics, the electrons would move as waves, and therefore would only form orbits where the wavelengths were whole numbers. If the wavelengths needed to complete an orbit was not a whole number, then the wavelength would cancel itself out when the electron’s trough met a peak on its way around.

Richard Feynman created the sum over histories theory to explain this. A particle is said to travel from A to B by every possible path. By adding up all the wavelengths for all the paths, and finding which cancel each other out, one can find the probability of traveling from A to B. This provides the math to predict particle movement, though in practice it is too difficult for calculating the movement of anything more than a simple atom.

Einstein’s general theory of relativity is considered a classical theory because it does not include quantum mechanics. This does not lead to inconsistency, though, as gravitational forces are so weak compared to other forces. But gravity would be much stronger in black holes or at the big bang, and as such needs to be integrated into quantum mechanics. Just as the idea of atoms collapsing was wrong, so too might be ideas of singularities. Scientists need to unify these two theories and already know some properties such a theory would have, as well as the areas in which it would have the greatest significance.